There is an ongoing need for an inexpensive and reliable coating apparatus which can be used to coat viscous fluids continuously and controllably on a wide web with excellent downweb and crossweb uniformity and controllable coating width. Coating die methods suffer from the drawback that it is difficult to fabricate a wide die with excellent crossweb thickness uniformity, and the cost and weight of the die become prohibitive as the width is increased. Rolling bank methods suffer from the drawbacks of being difficult to control in terms of both coating weight and coating width, and have a tendency toward coating thickness variability. The inability to precisely control coating width on a wide web leads to waste, e.g., in the form of uncoated web, material, spilled coating fluid, etc. Further, once such a coating process has been brought under control, changing any variable, such as the viscosity of the coating fluid, or the desired finished thickness of the coating, or the line speed, requires laborious trial-and-error changes to many of the other variables in order to regain control of the coating process.
The use of capillary tubes in a coating device is known. Many capillary tube coating methods involve providing a reservoir or rolling bank of coating fluid on the substrate being coated. This reservoir must be metered using an additional device, such as a nip roll. Control of such a system is difficult. U.S. Pat. No. 8,257,794 (Wang et al.) discloses another known capillary tube coating method which does not use a rolling bank wherein the capillary tube is first brought into contact with the substrate to be coated in order to commence the flow of the coating fluid, and then withdrawn to a precise distance from the surface in order to maintain the flow during coating. Such a system is also difficult to control. Other common drawbacks to known capillary tube coating methods is that they force a choice between droplet-wise dispensing of the coating fluid, or high-velocity high-volume jet flow, rather than permit continuous dispensing of a controlled flow.
The need exists for an inexpensive and reliable coating apparatus which can be used to coat viscous fluids continuously and controllably on a wide web with excellent downweb and crossweb uniformity and controllable coating width.